The present invention relates to a speed-difference-dependent hydraulic clutch, comprising a drive housing, which rotates in a housing that is fixed in space and forms a reservoir for a working fluid, a hydrostatic displacement machine, which is accommodated in said drive housing, a drive shaft, a friction clutch for connecting the drive shaft to the drive housing and a piston for acting on the friction clutch, a pressure being produced in a pressure space when a difference occurs between the space of the drive housing and that of the drive shaft, this pressure giving rise to action upon the friction clutch, and the hydrostatic displacement machine having a first and a second opening for the supply and discharge of working medium and a third and fourth opening for connection to the pressure chamber.
Clutches of this kind are used in various configurations in the drive train of motor vehicles for direct transmission of a torque or locking a linked differential for the purpose of driving the wheels on one axle or distributing the drive torque between two axles. The hydrostatic displacement machine comprises an inner rotor and an outer rotor, for example, but it is equally possible to use hydrostatic displacement machines of some other kind. In either case, two parts (e.g. the drive housing and the output shaft) perform a relative motion when there is a speed difference and thus produce a pressure that acts on the piston of the friction clutch.
A clutch of this kind is known, for example, from U.S. Pat. No. 5,536,215. In this, a valve formed by a bimetallic leaf is provided between the pressure space and a space at low pressure, the clutch space, this valve being intended to compensate for temperature-dependent changes in the viscosity of the working fluid. There are no further control members.
U.S. Pat. No. 4,727,966 has disclosed another clutch of this kind in a somewhat different arrangement. Here, outflow openings with restrictions are provided between the third and fourth opening and the pressure space, allowing different response pressures for the two possible directions of the speed difference, i.e. a distinction between driving and overrun mode and forward and reverse travel.
In one embodiment (FIG. 7), control members have furthermore already been provided, namely further outflow valves, the response pressure of which can be controlled. The actuating signal for these valves is determined by a controller from a variety of operating variables. The control members are accommodated in the rotating part and the actuating signals, which are hydraulic in this case, must therefore be supplied to them via a rotary input. No means of influencing the pressure on the intake side of the displacement machine is provided. All this and, in addition, the action of the centrifugal force on the control members renders sensitive, accurate and rapid control impossible.
DE 198 54 687 A has disclosed another clutch of this kind in yet another arrangement, in which it is not the displacement machine but the pressure chamber itself that is connected by a rotary input to a control valve, the latter being fixed to the housing, for pressure modulation on the delivery side. This allows only imperfect control. To tow the vehicle away, for instance, the displacement machine must be completely depressurized, and this requires a valve on the intake side as well. Moreover, the valve arrangement requires the entire clutch unit to be constructed in a way that has many other disadvantages.
It is therefore the object of the invention to develop a clutch of the generic type in such a way that sensitive, accurate and rapid control and adaptation of the characteristic is assured with the minimum outlay.
The foregoing object is achieved by virtue of the present invention wherein the first and the second opening are each connected in terms of flow to the reservoir, via a first and second rotary input and a first and second valve respectively, the valves being fixed to the housing, and that the first and second valve are designed as controllable throttle valves, with the result that a control action takes place both at the supply side and at the discharge side of the displacement machine, irrespective of the direction of the differential speed.
Control valves are thus provided on the delivery and on the intake side on the shortest path to an from the displacement machine, allowing the clutch to be acted upon directly with very little delay in all conceivable driving states. Since these are controllable throttle valves, which are therefore arranged in the main flow, control can be exercised sensitively and accurately over the entire actuating range.
In addition, there are all the advantages of control valves that are arranged fixed to a housing: simplicity of connection to the control unit, no adverse effects due to centrifugal force, no limitation in overall size and ease of access. It also allows the working medium to be supplied and discharged on one side of the displacement machine, allowing small-diameter rotary joints to be provided in a relatively simple manner. The additional outlay for the rotary leadthrough is balanced by the elimination of the nonreturn valves that would be required in the same line without the two control valves.
In a preferred embodiment, the first and second valve form a common control valve. Apart from the consequent reduction in cost, this also ensures enhanced reliability without any loss of functionality in driving operation. Here, the first and second valve can be proportional valves or be activated by a stepping motor.
As a development of the invention, a double-acting nonreturn valve is provided between the third and fourth opening and the pressure chamber. Since with this valve one side is always open, the pressure space can be depressurized very rapidly via this valve and the control valve, this being advantageous particularly in the case of braking (ABS, ESP). Moreover, the valve can be accommodated in a particularly space-saving manner in the wall between the displacement machine and the pressure space. Owing to the fact that it is only a single valve with a switching element (ideally with a ball), a satisfactory transition from one mode of operation to the other is assured.
If a cooling passage leads into the clutch space from the displacement machine via a throttle valve, this ensures adequate lubrication and cooling of the clutch, promoting a rapid and crisp response from the latter.
An advantageous detail of the invention comprises the common control valve being formed by an actuating piston that can be displaced in a bushing, the bushing being connected in terms of flow to the reservoir at both ends and having a first and a second lateral control opening, one of which is connected to the displacement machine by the first opening and the other is connected to the displacement machine by the second opening (delivery side and intake side respectively), and the actuating piston closing the first and the second control opening, respectively, in its two end positions and closing the two control openings reciprocally to a greater or lesser extent in the intermediate positions. As a result, the degree to which the two valves open and close is interdependent, this being achieved with the minimum outlay in terms of construction. Economy of space and accessibility are further improved if the bushing of the control valve is arranged in a plane transverse to the axis of rotation of the clutch, in the region of the rotary inputs.
Another advantageous detail of the invention comprises the double-acting nonreturn valve between the displacement machine and the pressure chamber comprising a tangential hole in a plane transverse to the axis of rotation of the clutch and a closing body guided in said hole, the two ends of the tangential hole being connected to the third and fourth opening of the displacement machine and their center being connected to the pressure space. The valve is thus unimpaired by centrifugal force, while taking up the minimum of space.